The present invention relates to an ink-printer capable of printing various output images on paper to satisfy a wide variety of needs, ranging from high-quality, high-speed printing in the printing industry to needs of office and personal printer fabrication industry and even to general-purpose economical, consumer output devices using various kinds of paper.
A conventional ink-printer as disclosed in Japanese Patent Laid-Open No. 167465/1981 and making use of an electrostatic force (Coulomb force) has been proposed. This printer has a lower plate on which print electrodes are arranged for individual dots. An insulative upper plate forms a single opening in the form of a slit. This opening is filled with ink. A counter electrode is placed opposite to the print electrodes through the opening such that a slight gap is left between the counter electrode and each print electrode. A pulsive voltage is applied between the counter electrode and each print electrode. Thus, a Coulomb force is applied to ink to squirt it toward the counter electrode. Therefore, the print head is simple in structure. Only a pattern of the print electrodes is required. Since the ink travels along the lines of electric force of the electric field produced between each print electrode and the counter electrode while the charged ink is undergoing a Coulomb force, it is not necessary to partition the opening in the print head by nozzles.
In this conventional printer, ink can be squirted from any arbitrary position even in the opening in the form of a slit. Furthermore, the structure is simple. Hence, the fabrication cost can be curtailed. However, the printer of this structure has the following problems.
(1) Before printing is started, a pulsed voltage pulse for producing a potential difference between mutually opposite electrodes is not applied. Therefore, the ink is hardly electrically charged. After a printing operation has progressed to some extent, or during the printing operation, electric charge is gradually accumulated on ink. Accordingly, the amount of electric charge on the ink differs between the initial state and a state in which a printing operation has progressed to some extent. Therefore, if a print pulse is applied, the Coulomb force exerted on the ink differs according to the state. Consequently, produced ink dots become nonuniform in size. Especially, under the initial state, ink droplets may not be squirted, resulting in dropouts. Alternatively, the dot diameters decrease. In this way, nonuniform ink dots are formed on the paper. As a result, the output image suffers from nonuniformities and thinned portions.
(2) When the print head is being driven at regular intervals, if a long interval time is established to permit a mechanical operation such as paper feeding or serial scan of the head, the amount of charge on the ink attenuates greatly. Therefore, if a print pulse is then applied, the amount of charge on the ink is insufficient in the same way as in (1). Consequently, a sufficient Coulomb force is not obtained. Ink droplets are not easily ejected. The output image suffers from dropouts and thinned portions. In this way, the print quality cannot be assured.
(3) Furthermore, electric charge left on the surface of the ink meniscus is attenuated during a quite short interval between successively applied print pulses. Therefore, the print pulses must always maintain an energy to permit ink to be squirted stably from this attenuated state. Consequently, the voltage value of the print pulses is set high, or the pulse width is set longer, to secure the necessary energy. For this reason, it has been difficult to reduce the driving voltage used for the print head. That is, it has been difficult to reduce the size of the power supply and to accomplish power savings. In addition, this has been a great obstacle in shortening the pulse width, i.e., in shortening the drive time. That is, increase in the print speed has been greatly hindered.
Accordingly, it is an object of the present invention to provide a small-sized printer which offers savings in power consumption, can provide an improved print speed, and can maintain high output print quality (i.e., free from nonuniformities and thinned portions due to dropouts).
An ink-printer in accordance with the present invention prints on a recording medium by extracting ink from an opening by an electrostatic force, the printer comprising: plural print electrodes arranged near the opening; a counter electrode located opposite to the print electrodes via a quite narrow gap; a driver circuit for selectively applying print pulses to the print electrodes; and a charge supply means for supplying desired electric charge to ink existing near the opening.
In one feature of the invention, there is further provided a conveyor mechanism for conveying the recording medium positioned between the counter electrode and the opening. When the recording medium is being scanned by the conveyor means, the charge supply means applies a uniform potential difference between each print electrode and the counter electrode, thus previously supplying desired electric charge to the ink in the opening.
In another feature of the invention, the charge supply means applies a voltage pulse that gives a uniform potential difference between each of the print electrodes and the counter electrode at least once immediately before application of the print pulses, thus previously supplying desired electric charge to the ink in the opening.